Effects of Cross and Self Fertilisation in the Vegetable Kingdom eBook

by which the stigma in the several genera is screened
from the action of the pollen from the same flower.
For instance, in Synaphea “the stigma is held
by the eunuch (i.e., one of the stamens which is barren)
safe from all pollution from her brother anthers,
and is preserved intact for any pollen that may be
inserted by insects and other agencies.”) As
far as anemophilous plants are concerned, we know
that they are apt to have their sexes separated, and
we can see that it would be an unfavourable circumstance
for them to bear their flowers very close to the ground,
as their pollen is liable to be blown high up in the
air (10/62. Kerner ‘Schutzmittel des Pollens’
1873 page 4.); but as the culms of grasses give sufficient
elevation, we cannot thus account for so many trees
and bushes being diclinous. We may infer from
our previous discussion that a tree bearing numerous
hermaphrodite flowers would rarely intercross with
another tree, except by means of the pollen of a distinct
individual being prepotent over the plants’
own pollen. Now the separation of the sexes,
whether the plant were anemophilous are entomophilous,
would most effectually bar self-fertilisation, and
this may be the cause of so many trees and bushes
being diclinous. Or to put the case in another
way, a plant would be better fitted for development
into a tree, if the sexes were separated, than if
it were hermaphrodite; for in the former case its
numerous flowers would be less liable to continued
self-fertilisation. But it should also be observed
that the long life of a tree or bush permits of the
separation of the sexes, with much less risk of evil
from impregnation occasionally failing and seeds not
being produced, than in the case of short-lived plants.
Hence it probably is, as Lecoq has remarked, that
annual plants are rarely dioecious.

Finally, we have seen reason to believe that the higher
plants are descended from extremely low forms which
conjugated, and that the conjugating individuals differed
somewhat from one another,—­the one representing
the male and the other the female—­so that
plants were aboriginally dioecious. At a very
early period such lowly organised dioecious plants
probably gave rise by budding to monoecious plants
with the two sexes borne by the same individual; and
by a still closer union of the sexes to hermaphrodite
plants, which are now much the commonest form. (10/63.
There is a considerable amount of evidence that all
the higher animals are the descendants of hermaphrodites;
and it is a curious problem whether such hermaphroditism
may not have been the result of the conjugation of
two slightly different individuals, which represented
the two incipient sexes. On this view, the higher
animals may now owe their bilateral structure, with
all their organs double at an early embryonic period,
to the fusion or conjugation of two primordial individuals.)
As soon as plants became affixed to the ground, their
pollen must have been carried by some means from flower
to flower, at first almost certainly by the wind,
then by pollen-devouring, and afterwards by nectar-seeking
insects. During subsequent ages some few entomophilous
plants have been again rendered anemophilous, and some
hermaphrodite plants have had their sexes again separated;
and we can vaguely see the advantages of such recurrent
changes under certain conditions.